The automotive, aerospace, consumer appliance, and other industries require adhesives that have a combination of properties such as high durability, resistance to continuous exposure to high temperatures, and good peel adhesion to a wide range of substrates such as metals, plastics, paints and the like. While rubber/resin based adhesives show excellent peel adhesion to many substrates, they perform poorly at elevated temperatures. Acrylic based adhesives, in contrast, either have outstanding peel adhesion and poor elevated temperature performance or low peel adhesion and outstanding elevated temperature performance. What is desired is a durable adhesive that is characterized by both good peel adhesion to a variety of substrates and a high shear adhesion failure temperature.
It is known that peel adhesion to various substrates is generally improved if acrylic pressure-sensitive adhesives are compounded with low molecular weight tackifying resins. For example, Japanese Patent 8569180 (assigned to Nitto) teaches the use of terpene phenolic resin as a tackifier in photocurable pressure-sensitive adhesives. The use of other types of resins, including xylene resin, is described in the Japanese Journal "Setchaku" (Vol. 23, p. 489-504, 1984). The use of cyclohexene resin is described in DE 323122 (1983). U.S. Pat. No. 4,418,120 to Kealy, et al., discloses a cross-linked tackified polymer based on isooctylacrylate and 3 to 7% by weight acrylic acid. The polymer is stated to have an inherent viscosity of 0.75 to 1.5 dl/g prior to cure. The cured product was stated to have shear value at 70.degree. C., of at least 5,000 minutes, a result which in practice has not been, to us, achievable.
Incorporating low molecular weight resins into the adhesive formulation typically results in adhesives which have poor high temperature shear adhesion. To minimize this, very high molecular weight polymers have heretofore been used for tackification. However, such polymers are difficult to manufacture on a commercial scale.
The durability, adhesiveness, and other properties of pressure-sensitive adhesives are also affected by both the choice of monomers which comprise the copolymer, and the type of curing process employed.
Japanese Patent No. 84-18774 discloses a radiation curable pressure-sensitive composition containing a terpene-phenolic resin and a glycidyl methacrylate monomer. The cure is through carbon-carbon double bonds and is activated by radiation.
U.S. Pat. No. 3,639,500 to Muny, et al., discloses a pressure-sensitive adhesive based on polyepoxide, an acrylic ester tackifier, and a diene polymer, which shows structural adhesive properties after being thermally activated. However, the room temperature shear properties of the adhesive before thermal activation are low due to the presence of high amounts of low molecular weight polyepoxide. Similarly, U.S. Pat. No. 4,456,741 to Ames discloses a hot melt acrylic adhesive containing tackifiers having good peel adhesion and permanent tack, but shear adhesion is low.
U.S. Pat. No. 4,726,982 to Traynor, et al., discloses a tackified acrylic based pressure-sensitive composition containing N-vinyl-2-pyrrolidone, and described as being useful in adhering to high solids automotive paints. U.S. Pat. No. 3,903,057 to Gardner et al., and U.S. Pat. No. 3,893,982 to Gardner et al., describe a pressure-sensitive adhesive comprising a copolymer containing an acid-epoxy monomer system formulated with 1,3-bis(dimethylamino)-2-hydroxypropane. The composition is cured during solvent evaporation, due to the presence of the amine catalyst. Similarly, U.S. Pat. No. 4,404,246 to Charbonneau, et al., describes an acrylic tape produced by on-web polymerization in which a latent cross-linking agent like a lower alkoxylated aminoformaldehyde is added to achieve high cohesive strength and peel adhesion after baking.
Despite the plethora of monomer-monomer and copolymer-tackifier combinations disclosed in the prior art, no one has heretofore taught how to make an acrylic based pressure-sensitive adhesive characterized by a high shear adhesion failure temperature and good peel adhesion on a wide range of substrates. Nor have the benefits of dual cure cross-linking, as will be explained herein, been appreciated if obtained at all.